Increases in luminous efficiencies and diode luminous intensity have made possible the use light emitting diodes (LEDs) for general lighting and illumination applications. As the efficiency and intensity of high brightness LED devices (HB LEDs) approaches those of incandescent light sources, the packaging of LEDs for general lighting and control of the light radiation patterns and spectral viewing angles becomes increasingly important. In order to replace a tungsten filament bulb, a diode package must facilitate heat dissipation and expand both the luminous viewing angle and the radiation pattern.
Incandescent filament light bulbs emit light at angles in the range 180 to −180 degrees about the bulb. However, basic LEDs emit only a 90 degree cone of light. Various techniques such as diffusion on the LEDs and combining plural LEDs have increased the viewing angle. See for example, U.S. Pat. No. 5,122,781 to Saubolle and U.S. Pat. No. 5,140,220 to Hasegawa. However, these techniques include cumbersome optics and add manufacturing and material costs without achieving the desired 180 to −180 degree viewing angle.
A packaging challenge arises because the package must both conduct heat away from the LEDs and at the same time permit unobstructed emission. While several packaging technologies have been developed for a single LED, most dispose the LED within a cavity and use a reflector cup to guide the light out. Some photons emitted from the side of the LED die strike the cavity walls and never reflect out. Similarly when multiple LEDs are placed adjacent each other on a substrate, photons reflected between adjacent diodes are never recovered. Each LED die is typically a rectangular parallepiped that emits light from each of its six surfaces. When the LEDs are placed in rows and columns inside a cavity or on a planar conductive substrate, there is a significant photon absorbance and obstruction by the adjacent dies. Accordingly, there remains a need for an LED assembly that can emulate the light distribution created by an incandescent filament bulb.